/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null||l2 == null) {
            return null;
        }
        ListNode head = null,tail = null;
        int sum = 0;
        int count = 0;
        while(l1!=null||l2!=null) {
            sum = (l1 == null ?0:l1.val) + (l2 == null?0:l2.val);
            if(count>0) {
                sum+=count;
            }
            count = sum/10;
            if(head == null) {
                head = tail = new ListNode(sum%10);
            }else {
                tail.next = new ListNode(sum%10);
                tail = tail.next;
            }
            if(l1!=null) {
                l1 = l1.next;
            }
            if(l2!= null) {
                l2 = l2.next;
            }
        }
        if(count>0) {
            tail.next = new ListNode(count);
            tail = tail.next;
        }
        return head;
    }
}